Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
Evaporative leaks may be identified using engine-off natural vacuum (EONV) during key-off conditions when a vehicle engine is not operating. Therein, correlations between temperature and vacuum build-up are advantageously used to detect fuel system leaks. In particular, a fuel system is isolated at key-off, and as a fuel tank cools down, a vacuum is generated therein. Vacuum generation is monitored over a long time, and based on a rate of subsequent vacuum bleed-up, a leak can be identified. Another approach for leak detection during key-off conditions is shown by Siddiqui in U.S. Pat. No. 8,074,627. Therein, a fuel pump is operated to store vacuum in an accumulator. The stored vacuum is then applied on the fuel system during a key-off condition to identify a leak.
The inventors herein have identified a potential issue with such approaches. In these approaches, temperature (of the fuel tank) is not only a control factor but also a noise factor. For example, the EONV approaches rely on a correlation between fuel tank temperature and pressure to generate and apply vacuum on the fuel tank. However, depending on how long a vehicle engine was on before the leak test was initiated (which affects how much heat was rejected from the running engine to the fuel tank), a temperature of the parking surface where the vehicle is parked, as well as wind and sun loading on the fuel system, leak test results may vary. The same factors may likewise corrupt pressure data collected in the approach of Siddiqui. Consequently, in either approach, false failures or false passes may occur, degrading exhaust emissions. The problem may be exacerbated in hybrid vehicles where engine run times are low such that heat rejection to the fuel tank during engine operation is also low. Consequently, a temperature drop in the fuel tank during the key-off may not be enough to generate sufficient EONV for a leak test.
In one example, the above issue may be at least partly addressed by a method for a vehicle fuel system, comprising: during a vehicle-off condition, and while a fuel tank temperature stays within a threshold range, operating a fuel pump to raise a fuel tank vapor pressure to identify leaks in the fuel system. In this way, fuel system leaks can be performed with reduced noise contribution from fuel tank temperatures.
For example, a vehicle powertrain control module (PCM) may be set to a sleep mode in response to a vehicle-off event (e.g., a key-off event). The PCM may then we woken up after a first duration (e.g., in hours) has elapsed since the key-off event. As such, the first duration may be sufficiently long such that fuel tank temperatures and pressures are expected to have stabilized by the time the PCM is woken up. The PCM may seal the fuel system upon waking up and monitor changes in fuel tank temperature and/or pressure for a second duration that is shorter than the first duration (e.g., in seconds). If there is no substantial change in fuel tank temperature over the second duration (e.g., the fuel tank temperature remains within a range), it may be assumed that if a leak test is performed, a temperature contribution to noise during the diagnostics may be substantially low (or negligible). Accordingly, a fuel pump coupled to the fuel tank may be operated to initiate a leak test. By operating the fuel pump, fuel in the fuel tank is agitated, causing a fuel vapor pressure to increase. That is, a number of moles of fuel in the vapor space of the fuel tank is increased, thereby increasing a fuel tank pressure. Following the fuel tank pressure build-up, pump operation is discontinued, and a rate of pressure decay or bleed-down is monitored and compared to a threshold rate. The threshold rate may be calibrated for the fuel tank temperature. Additionally, the threshold rate may be calibrated to compensate for fuel level, altitude, and fuel type. The presence of a leak may be indicated based on bleeding down of the fuel tank pressure at a faster rate (e.g., faster than the threshold rate).
In this way, the principles of an ideal gas law may be advantageously used to perform an engine-off leak test without relying on temperature as a control factor. By operating a fuel pump during vehicle off conditions when fuel tank temperatures are stable, a number of moles of fuel vapor in a fuel tank can be increased and a relation between the moles of fuel vapor and a fuel tank pressure can be advantageously used to identify fuel system leaks. By reducing the reliance on temperature as a control factor in the fuel system leak test, temperature-induced noise factors in a leak test can also be reduced. In addition, an engine-off leak test can be reliably and accurately performed even in vehicles, such as hybrid vehicles, where there is reduced heat rejection to a fuel tank due to infrequent engine operation. By performing an active leak test that is based on the molar fuel content of fuel vapor rather than an opportunistic leak test that is based on the temperature of fuel vapor, the frequency of running and completing a leak test is improved. By improving leak detection, the quality of exhaust emissions and likelihood of emissions compliance is improved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.